Add test for focus_node.0.dart API example. (#146943)

This PR contributes to flutter/flutter#130459

### Description
- Adds `examples/api/test/widgets/focus_manager/focus_node.0_test.dart` test

Author: ksokolovskyi

dev/bots/check_code_samples.dart

@@ -392,7 +392,6 @@ final Set<String> _knownMissingTests = <String>{
   'examples/api/test/widgets/navigator/restorable_route_future.0_test.dart',
   'examples/api/test/widgets/navigator/navigator_state.restorable_push.0_test.dart',
   'examples/api/test/widgets/focus_manager/focus_node.unfocus.0_test.dart',
-  'examples/api/test/widgets/focus_manager/focus_node.0_test.dart',
   'examples/api/test/widgets/framework/build_owner.0_test.dart',
   'examples/api/test/widgets/framework/error_widget.0_test.dart',
   'examples/api/test/widgets/inherited_theme/inherited_theme.0_test.dart',

examples/api/test/widgets/focus_manager/focus_node.0_test.dart

@@ -0,0 +1,191 @@
+// Copyright 2014 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'package:flutter/material.dart';
+import 'package:flutter/services.dart';
+import 'package:flutter_api_samples/widgets/focus_manager/focus_node.0.dart'
+    as example;
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  testWidgets(
+    'FocusNode gets focused and unfocused on ColorfulButton tap',
+    (WidgetTester tester) async {
+      await tester.pumpWidget(
+        const example.FocusNodeExampleApp(),
+      );
+
+      final Element button = tester.element(
+        find.byType(example.ColorfulButton),
+      );
+
+      expect(
+        tester.binding.focusManager.primaryFocus?.context,
+        isNot(equals(button)),
+      );
+
+      // Tapping on ColorfulButton to focus on FocusNode.
+      await tester.tap(find.byType(example.ColorfulButton));
+      await tester.pump();
+
+      expect(
+        tester.binding.focusManager.primaryFocus?.context,
+        equals(button),
+      );
+
+      // Tapping on ColorfulButton to unfocus from FocusNode.
+      await tester.tap(find.byType(example.ColorfulButton));
+      await tester.pump();
+
+      expect(
+        tester.binding.focusManager.primaryFocus?.context,
+        isNot(equals(button)),
+      );
+    },
+  );
+
+  testWidgets(
+    'FocusNode updates the text label when focused or unfocused',
+    (WidgetTester tester) async {
+      await tester.pumpWidget(
+        const example.FocusNodeExampleApp(),
+      );
+
+      expect(find.text('Press to focus'), findsOneWidget);
+      expect(find.text("I'm in color! Press R,G,B!"), findsNothing);
+
+      // Tapping on ColorfulButton to focus on FocusNode.
+      await tester.tap(find.byType(example.ColorfulButton));
+      await tester.pump();
+
+      expect(find.text('Press to focus'), findsNothing);
+      expect(find.text("I'm in color! Press R,G,B!"), findsOneWidget);
+
+      // Tapping on ColorfulButton to unfocus from FocusNode.
+      await tester.tap(find.byType(example.ColorfulButton));
+      await tester.pump();
+
+      expect(find.text('Press to focus'), findsOneWidget);
+      expect(find.text("I'm in color! Press R,G,B!"), findsNothing);
+    },
+  );
+
+  testWidgets(
+    'FocusNode updates color of the Container according to the key events when focused',
+    (WidgetTester tester) async {
+      await tester.pumpWidget(
+        const example.FocusNodeExampleApp(),
+      );
+
+      // Tapping on ColorfulButton to focus on FocusNode.
+      await tester.tap(find.byType(example.ColorfulButton));
+      await tester.pump();
+
+      final Finder containerFinder = find.descendant(
+        of: find.byType(example.ColorfulButton),
+        matching: find.byType(Container),
+      );
+
+      Container container = tester.widget<Container>(containerFinder);
+      expect(container.color, equals(Colors.white));
+
+      await tester.sendKeyEvent(LogicalKeyboardKey.keyR);
+      await tester.pump();
+
+      container = tester.widget<Container>(containerFinder);
+      expect(container.color, equals(Colors.red));
+
+      await tester.sendKeyEvent(LogicalKeyboardKey.keyG);
+      await tester.pump();
+
+      container = tester.widget<Container>(containerFinder);
+      expect(container.color, equals(Colors.green));
+
+      await tester.sendKeyEvent(LogicalKeyboardKey.keyB);
+      await tester.pump();
+
+      container = tester.widget<Container>(containerFinder);
+      expect(container.color, equals(Colors.blue));
+    },
+  );
+
+  testWidgets(
+    'FocusNode does not listen to the key events when unfocused',
+    (WidgetTester tester) async {
+      await tester.pumpWidget(
+        const example.FocusNodeExampleApp(),
+      );
+
+      final Finder containerFinder = find.descendant(
+        of: find.byType(example.ColorfulButton),
+        matching: find.byType(Container),
+      );
+
+      Container container = tester.widget<Container>(containerFinder);
+      expect(container.color, equals(Colors.white));
+
+      await tester.sendKeyEvent(LogicalKeyboardKey.keyR);
+      await tester.pump();
+
+      container = tester.widget<Container>(containerFinder);
+      expect(container.color, equals(Colors.white));
+
+      await tester.sendKeyEvent(LogicalKeyboardKey.keyG);
+      await tester.pump();
+
+      container = tester.widget<Container>(containerFinder);
+      expect(container.color, equals(Colors.white));
+
+      await tester.sendKeyEvent(LogicalKeyboardKey.keyB);
+      await tester.pump();
+
+      container = tester.widget<Container>(containerFinder);
+      expect(container.color, equals(Colors.white));
+    },
+  );
+
+  testWidgets(
+    'FocusNode sets color to the white when unfocused and sets it back to the selected one when focused',
+    (WidgetTester tester) async {
+      await tester.pumpWidget(
+        const example.FocusNodeExampleApp(),
+      );
+
+      final Finder containerFinder = find.descendant(
+        of: find.byType(example.ColorfulButton),
+        matching: find.byType(Container),
+      );
+
+      Container container = tester.widget<Container>(containerFinder);
+      expect(container.color, equals(Colors.white));
+
+      // Tapping on ColorfulButton to focus on FocusNode.
+      await tester.tap(find.byType(example.ColorfulButton));
+      await tester.pump();
+
+      container = tester.widget<Container>(containerFinder);
+      expect(container.color, equals(Colors.white));
+
+      await tester.sendKeyEvent(LogicalKeyboardKey.keyR);
+      await tester.pump();
+
+      container = tester.widget<Container>(containerFinder);
+      expect(container.color, equals(Colors.red));
+
+      // Tapping on ColorfulButton to unfocus from FocusNode.
+      await tester.tap(find.byType(example.ColorfulButton));
+      await tester.pump();
+
+      container = tester.widget<Container>(containerFinder);
+      expect(container.color, equals(Colors.white));
+
+      // Tapping on ColorfulButton to focus on FocusNode.
+      await tester.tap(find.byType(example.ColorfulButton));
+      await tester.pump();
+
+      container = tester.widget<Container>(containerFinder);
+      expect(container.color, equals(Colors.red));
+    },
+  );
+}